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A2 CHEMISTRY NOTES – Transition Metals WHAT IS A TRANSITION METAL? The terms transition metal (or element) and __ block element are sometimes used as if they mean the same thing. They don't: d block elements The 3s and 3p levels are full, but next the ___ level fills instead to give potassium and then calcium. Only after that do the 3d levels fill. The elements in the Periodic Table which correspond to the d levels filling are called _ ______ ________. Sc Ti V Cr Mn Fe Co Ni Cu Zn [Ar] 3d14s2 You will notice that the pattern of filling isn't entirely tidy! It is broken at both chromium and copper. [Ar] 3d104s2 A transition metal is one, which forms one or more stable ions, which have _______________ __________ __ __________ 1 A2 CHEMISTRY NOTES – Transition Metals Transition metal ions The ___ orbital is filled _______ the 3d orbitals as it is initially lower in energy. The 4s electrons then become higher in energy and are the _________ electrons 4s 3d 4s 3d Not all d block elements count as transition metals! Scandium and _____ are d-block metals but not transition metals. Scandium has the electronic structure [Ar] ________. When it forms ions, it always loses the __ ________ electrons and ends up with an argon structure. The Sc3+ ion has __ __ __________ and so doesn't meet the definition. Zinc has the electronic structure [Ar] ________. When it forms ions, it always loses the two 4s electrons to give a 2+ ion with the electronic structure [Ar] ____. The zinc ion has ___ __ _________ and doesn't meet the definition either. By contrast, copper, [Ar] ______, forms two ions. In the Cu+ ion the electronic structure is [Ar] _____. However, the more common Cu2+ ion has the structure [Ar] ____. Copper is definitely a ____________ metal because the Cu2+ ion has an __________ d shell. Remember this: When d-block elements form ions, the 4s electrons are lost ______. To write the electronic structure for Co2+: Co Co2+ The 2+ ion is formed by the loss of the two 4s electrons To write the electronic structure for V3+: V V3+ The 4s electrons are lost first followed by one of the 3d electrons Transition metals have specific properties and these are: 1. Variable oxidation state (number) 2. The formation of complex ions 3. The formation of coloured compounds 4. Catalytic activity 2 A2 CHEMISTRY NOTES – Transition Metals VARIABLE OXIDATION STATE (NUMBER) Transition metal chemistry show a wide range of oxidation numbers. Other than transition metals only LEAD and TIN show variable oxidation state to any extent Sc Ti V Cr Mn Fe Co Ni Cu Zn Explaining the variable oxidation states in the transition metals Usually when a metal forms the compound forms the most stable compound which is the one in which most _______ is released. The more energy released, the more ______ the compound. The key energy changes are: The amount of energy needed to _______ the metal The amount of energy released when the compound forms. This will either be ________ enthalpy if you are thinking about solids, or the ____________ enthalpies of the ions if you are thinking about solutions. The more highly charged the ion, the more ____________ you have to remove and the more __________ energy you will have to provide. But off-setting this, the more highly charged the ion, the more energy is _________ either as ________ enthalpy or the ___________ enthalpy of the metal ion. Transition metals have variable oxidation states as the 4s and 3d electrons have _________ energies. 3 A2 CHEMISTRY NOTES – Transition Metals Redox Titrations Manganate (VII) Potassium permanganate is an oxidising agent and has a rich _________ colour. If it is reduced in the presence of dilute ____, usually ________ acid it becomes Mn2+, a ____ solution and so when it is used in a titration it is self indicating. In the burette: ___________________________ In the conical flask: _______________________________ End point: Purple colour of permanganate ______ persists. Oxidation half equation: Reduction half equation: Full Equation: Mole Ratio: _______ Dichromate (VI) Acidified potassium dichromate is an oxidizing agent and it is _______ in colour. If it is reduced in the presence of dilute ________, usually _________ acid it becomes a ______ solution of ____. This end point would be difficult to see so an indicator is used. In the burette: _________________________ In the conical flask: ________________________ End point: Depends on the _________________. Oxidation half equation: Reduction half equation: Full Equation: Mole Ratio: _______ 4 A2 CHEMISTRY NOTES – Transition Metals Thiosulphate A solution of thiosulphate ions (S2O32-) acts as a reducing agent. It is a ______ solution and is often used to find out the concentration of _________ solutions. The iodine is reduced to _______ and the thiosulphate oxidized to tetrathionate When nearing the end point of these titrations is reached, and the brown colour of the iodine fades to _______, a ______ indicator is added. This forms a ____-_______ colour with iodine. The ____-_______ colour just disappears as the end point of the titration is reached In the burette: _____________________ In the conical flask: ___________ (starch indicator added when colour fades to yellow) End point: __________________________________________________ Oxidation half equation: Reduction half equation: Full Equation: Mole Ratio: ______ 5 A2 CHEMISTRY NOTES – Transition Metals FORMATION OF COMPLEX IONS A complex ion has a metal ion at its centre with a number of other molecules or ions called ligands surrounding it. Ligands use their _____ pairs of electrons to form __-___________ bonds with the metal ion. All ligands are lone pair _______. The ligand is acting as a Lewis _______ – electron pair ______ The metal ion is acting as a Lewis ______ - electron pair ________ A ligand is a molecule or ions that donate an ________ pair to a central metal ion thus forming a __-__________ bond Co-ordination Number Co-ordination number is the number of pairs of ________ donated to the central metal ion and is usually 2, 4 or 6. Unidentate ligands donate one electron pair water, ammonia and halide ions. H2O and NH3 are uncharged and similar in ____ Bidentate ligands donate ____ electron pairs H2NCH2CH2NH2, C2O42- Multidentate ligands [Ni (NH2CH2CH2NH2)3] 2+ [Cr (C2O4)3] 3- [Cu(EDTA)]2- Co-ordination number of these complexes is __ Complexes containing bidentate and multidentate ligands are called ____________. ___________ are more stable than complexes and the chellation process has positive __________. 6 A2 CHEMISTRY NOTES – Transition Metals THE SHAPES OF COMPLEX METAL IONS linear [Ag(NH3)2]+ 2-co-ordinated complex ions tetrahedral [CrCl4]4-co-ordinated complex ions square planar [Pt(NH3)2Cl2] 4-co-ordinated complex ions octahedral [Co(NH3)6]3+, [Cr(H2O)6]2+ and other aqua complexes 6-co-ordinated complex ions 7 A2 CHEMISTRY NOTES – Transition Metals NAMING COMPLEX METAL IONS The names of complex ions are long, but formulae are simply systematic in much the same way that organic names are systematic. The table shows the code for some ligands, multipliers and for metal anions. ligand H2O NH3 OHCl- coded by AQUA AMMINE HYDROXO CHLORO no of ligands coded by 2 3 4 5 6 DI TRI TETRA PENTA HEXA Metals in anionic changed to complexes cobalt COBALTATE aluminium ALUMINATE chromium CHROMATE vanadium VANADATE copper CUPRATE iron FERRATE Only one type of ligand: E.g [Cu(H2O)6]2+ is called the __________copper(II) ion. two "a"s next to each other in the name is OK) More than one type of ligand in an ion: The ligands are named in ___________ order - ignoring the prefixes. [Cu(NH3)4(H2O)2]2+ is called the _____________________(II) ion. The "ammine" is named before the "aqua" because "am" comes before "aq" in the alphabet. The "tetra" and "di" are ignored. Naming the metal For positively charged complex ions A positively charged complex ion is called a _________ complex. The metal in this is named exactly as you would expect, with the addition of its oxidation state. E.g. [Cu(H2O)6]2+ is called the _______________(II) ion because the copper's oxidation state is +2. For negatively charged complex ions A negatively charged complex ion is called an ___________ complex.. In this case the name of the metal is modified to show that it has ended up in a negative ion. This is shown by the ending -ate. With many metals, the basic name of the metal is changed as well - sometimes drastically! Common examples include: [CuCl4]2-. _____________________________ ion. [Al(H2O)2(OH)4]- ___________________________ ion. 8 A2 CHEMISTRY NOTES – Transition Metals Some Important Complexes Cisplatin Cis-platin Use: Side effects: trans[PtCl2(NH3)2] As an __________ drug nausea, vomiting, allergic reactions, hearing loss, kidney damage Hemoglobin The Role of Fe2+ The haem part of haemoglobin is a ___ (II) complex with a multidentate ligand. The Fe2+ is central to the action of ____ blood cells in carrying ___________. Why is carbon monoxide toxic? Oxygen binds ___________ to the Fe2+ ion at the centre of the hemoglobin complex. Carbon monoxide, however binds strongly to the complex and thus prevents ________ being carried round the body. 9 A2 CHEMISTRY NOTES – Transition Metals THE COLOURS OF COMPLEX METAL IONS Why is copper(II) sulphate solution blue? If white light passes through copper(II) sulphate solution, some wavelengths are absorbed by the solution. Copper(II) ions in solution absorb light in the ___ region of the spectrum. The light which passes through the solution and out the other side will have all the colours in it except for the ___. We see this mixture as _____, the complementary colour of ___. This is a mixture of the remaining wavelengths The formation of coloured compounds When the ligands bond with the transition metal ion the d electron energy levels _____ into two groups. When white light is passed through a solution of this ion, some of the energy in the light is used to promote an _______ from the lower set of orbitals into a space in the upper set. The ______ light that is absorbed leaves other colours to pass through. ΔE = h Where ΔE is the enery ___ between the split d orbitals h is Planks constant (6.626 x 10-34Js) is the frequency of light absorbed in Hz Attaching ligands to a metal ion splits the energies of the d orbitals. Light is absorbed as electrons move up between one __ orbital and another. Some common examples The size of the energy gap between them varies with the nature of the transition ______ ion, its __________ state, the __-_________ number and the nature of the ____________. 10 A2 CHEMISTRY NOTES – Transition Metals Colorimetry A spectrometer measures the degree of absorbance of a solution and can be used to find the ______________ of a solution. a filter is used which is usually a _____________ colour to the solution to make the readings more sensitive. a ____________ curve plotted with samples of ________ concentrations. 11 A2 CHEMISTRY NOTES – Transition Metals CATALYTIC ACTIVITY Transition metals and their compounds function as catalysts either because of their ability to change ____________ state or, in the case of the metals, to adsorb other substances on to their __________ and activate them in the process. Transition metals as heterogeneous catalysts Heterogeneous catalysts are in a different _______ to the reactants. The reaction occurs on the ________ of the catalyst and provides an _____________ route with a _________ activation energy Use of Catalyst Supports Transition metal catalysts can be very costly and since it only the ________ that is the active part of the catalyst a support can be used on which the transition metal is coated. The support provides a large ________ ______. Poisoning of Catalysts There are some substrates that bind strongly to the catalyst surface and blocks the ________ sites. This is called poisoning of the catalyst. It happens with sulphur particles in the Haber Process and lead particles can do the same with the palladium catalyst in catalytic converters. This means that the catalyst is not working as efficiently as it should and will have implications in terms of the overall _____ of the process as it may take ________ and produce _____ product. 12 A2 CHEMISTRY NOTES – Transition Metals Iron in the Haber Process 2N2(g) + 3H2(g) The Haber Process combines hydrogen and nitrogen to make ___________ using an _____ catalyst. 2NH3(l) Iron Chromium oxide in the production of methanol Methanol is used a _____ and a ________ and it can be produced from reaction of carbon monoxide and hydrogen. Vanadium(V) oxide in the Contact Process At the heart of the _________ Process is a reaction which converts sulphur dioxide into sulphur trioxide. Sulphur dioxide gas is passed together with air (as a source of oxygen) over a solid vanadium(V) oxide catalyst. The ability of vanadium to change it’s _________ state allows it to catalyse this reaction CO(g) + 2H2(g) CH3OH(l) Cr2O3 2SO2(g) + ½ O2(g) SO2(g) + V2O5(s) V2O5 2SO3(g) SO (l) + V O (s) 3 2 4 The sulphur dioxide is oxidised to sulphur _________ by the vanadium(V) oxide. In the process, the vanadium(V) oxide is _________ to vanadium(IV) oxide. The vanadium(IV) oxide is then re-oxidised by _______. Although the catalyst has been temporarily changed during the reaction, at the end it is chemically the same as it started. 13 V2O4(s)+ ½ O2(g) V O (s) 2 5 A2 CHEMISTRY NOTES – Transition Metals Transition metalcompounds as homogeneous catalysts Homogeneous catalysts are in the same ______ as the reactants and work by forming an intermediate state with a different oxidation number. This route has a _______ activation energy than the uncatalysed route Fe3+ catalyst Iron(II) ions act as a homogenous catalyst for the reaction between peroxodisulphate and iodide ions S2O82-(aq) + 2eFe3+(aq) + eI2(aq) + 2e2- 2SO42-(aq) +2.01 Fe2+ (aq) +0.77 2I (aq) +0.54 The oxidation of Fe2+ -by the peroxodisulphate will be the first stage of the reaction S2O82-(aq) + 2Fe2+(aq) 2Fe3+ (aq) + 2SO42-(aq) E = _____________ The Fe3+ ions are then reduced by the iodide ions 2I-(aq) + 2Fe3+(aq) 2Fe2+ (aq) + I2(aq) E = _____________ Autocatalysis The manganate (VII) ions produce Mn2+, carbon dioxide gas and water when it reacts with acidified aqueous ethandiote ions. (notice that oxidizing and reducing agents are both negatively charged which makes a straightforward route kinetically unfavoured.) The reaction is initially slow but gets faster as the Mn2+ produced catalyses the reaction as follows: The ethanedioate ion then reduces the Mn3+ ion 5Mn3+(aq) + 5C2O42-(aq) 5Mn2+(aq) + 10CO2(g) 14 A2 CHEMISTRY NOTES – Transition Metals Hexaqua ions (6 water ligands around a central ion) The positive charge on the transition metal ions attract _____ water molecules. The ions have vacant _______ which allow a __-_________ bond to form. The small size of the water molecules means there is room for __ waters around the ion. M2+ [Cu(H2O)6]2+ [Co(H2O)6]2+ [Fe(H2O)6]2+ Colour Colour 3+ [Al(H2O)6] [Cr(H2O)6]3+ [Fe (H2O)6]3+ The acidity of the hexaaqua (III) ions: Hydrolysis Reactions Cr3+ 3+ ions formed in solution are ________ and have a pH in the ___- ___ range. The small ______ and the large _______ make the M3+ ion very _________. A _________ ion is lost from one of the ligand water molecules as the M3+ ion pulls electron density away from the O-H bond in the water ligands. The complex ion is acting as an ______ by donating a hydrogen ion to water molecules in the solution. The water is acting as a ________ by accepting the hydrogen ion. For chromium For Iron For Aluminium The acidity of the hexaaqua (II) ions The +2 ions are not as polarizing as they have larger radii and smaller charges and so they are not able to pull electron density from the O-H bond as strongly as the +3 ions. These complexes are less acidic. For Iron For Cobalt For Copper 15 A2 CHEMISTRY NOTES – Transition Metals Hydrolysis Reactions Of Hexaaqua Metal Ions With Hydroxide Ions The hydroxide ion removes a hydrogen ion from one of the ligand water molecules. This is an hydrolysis reaction. Adding OH-(aq) ions to 2+ hexaaqua ions 16 Adding OH-(aq) to 3+ hexaaqua ions A2 CHEMISTRY NOTES – Transition Metals Reaction of complex ions with NaOH(aq) hexaaquacobalt(II) hexaaquacopper(II) hexaaquairon(II) Hexaaquaaluminium hexaaquairon(III) hexaaquachromium(III) Reversing the process If adding hydroxide ions removes hydrogen ions from the hexaaqua complex one at a time, it doesn't seem unreasonable that you could put them back again by adding an ______. That's just what happens! 17 A2 CHEMISTRY NOTES – Transition Metals Reactions Of Hexaaqua Metal Ions With Carbonate Ions The general case for 2+ ions These have the form [M(H2O)6]2+. They are only very weak _______ and so when the carbonate ions are added they form the metal(II) carbonate which is _______ in water: [M(H2O)6]2+(aq) + CO32-(aq) MCO3(s) + 6H2O(l) Copper (II) [Cu(H2O)6]2+(aq) + CO32-(aq) CuCO3(s) + __H2O(l) _______ __________ Cobalt (II) [Co(H2O)6]2+(aq) + CO32-(aq) CoCO3(s) + __H2O(l) _______ __________ Iron (II) [Fe(H2O)6]2+(aq) + CO32-(aq) FeCO3(s) + __H2O(l) _______ __________ The general case for 3+ ions These have the form [M(H2O)6]3+. They are able to act as ________. [M(H2O)6]3+(aq) +3H2O(l) [M(H2O)3(OH)3](s) + 3H3O+ (aq) In this case the carbonate react as a base with two H3O+(aq) to form ____ and water CO32-(aq) + 2H3O+(aq) 3H2O(l) + CO2(g) Overall the reaction is 2[M(H2O)6]3+(aq) + 3CO32-(aq) [M(H2O)3(OH)3](s) + 3H2O(l) +3CO2 Aluminium 2[Al(H2O)6]3+(aq) + 3CO32-(aq) 2[Al(H2O)3(OH)3](s)+ 3H2O(l) +3CO2(g) _______ __________ Chromium 2[Cr(H2O)6]3+(aq) + 3CO32-(aq) 2[Cr(H2O)3(OH)3](s)+ 3H2O(l) +3CO2(g) _______ __________ Iron(III) 2[Fe(H2O)6]3+(aq) + 3CO32-(aq) 2[Fe(H2O)3(OH)3](s)+ 3H2O(l) +3CO2(g) _______ __________ 18 A2 CHEMISTRY NOTES – Transition Metals Amphoteric hydroxides An amphoteric substance has both _______ and _____ properties. In other words, it will react with both bases and acids. Some of the metal hydroxides we've been looking at are doing exactly that. Chromium(III) hydroxide as an amphoteric hydroxide With bases [Cr(H2O)3(OH)3] (s) + 3OH-(aq) [Cr(OH)6]3-(aq) + 3H2O(l) With acids [Cr(H2O)3(OH)3] (s) + 3H3O+-(aq) [Cr(H2O)6]3+(aq) + 3H2O(l) Aluminium hydroxide as an amphoteric hydroxide With bases [Al(H2O)3(OH)3] (s) + OH-(aq) [Al(OH)4]-(aq) + 3H2O(l) With acids [Al(H2O)3(OH)3] (s) + 3H3O+-(aq) [Al(H2O)6]3+(aq) + 3H2O(l) 19 A2 CHEMISTRY NOTES – Transition Metals Ligand Exchange Reactions with Ammonia This type of reaction involves breaking the co-ordinate bond and replacing it with a co-ordinate bond from a different ligand. Ammonia is similar in size to water and so the final product is a hexammine complex. The initial reaction of the however is a hydrolysis reaction to form the hydroxide precipitate. Some examples suggested by the syllabus: Cobalt (+2 ion) [Co(H2O)6]2+ + 2NH3(aq) [Co(H2O)4(OH)2 ](s) + 2NH4+(aq) When excess ammonia is added the hydroxide precipitate dissolves and the H2O ligands are replaced by the NH3 ligands [Co(H2O)6]2+ + 6NH3(aq) [Co(NH3)6]2+(aq) + 6H2O(l) However this substitution process can be incomplete and any of the following complexes could be in existence in solution: [Co(H2O)5(NH3) ], [Co(H2O)4(NH3)2 ], [Co(H2O)3(NH3)3 ], [Co(H2O)2(NH3)4], [Co(H2O) (NH3)4] Chromium (+3 ion) [Cr(H2O)6]3+ + 3NH3(aq) [Cr(H2O)3(OH)3 ](s) + 3NH4+(aq) With excess ammonia [Cr(H2O)6]3+ + 6NH3(aq) [Cr(NH3)6]3+ (aq) + 6H2O(l) Copper (+2 ion that does not get fully substituted) [Cu(H2O)6]2+ + 2NH3(aq) [Cu(H2O)4(OH)2 ](s) + 2NH4+(aq) Diagram of the tetraamminediaquacopper (II) complex With excess ammonia just 4 of the water ligands are replaced [Cu(H2O)6]2+ + 4NH3(aq) [Cu(H2O)2 (NH3)4]2+(aq) + 4H2O(l) Complex Colour 2+ [Co(NH3)6] (aq) [Cr(NH3)6]3+ (aq) [Cu(H2O)2 (NH3)4]2+(aq) 20 A2 CHEMISTRY NOTES – Transition Metals Ligand Exchange Reactions with Chloride ions Chlorine ions are larger than water and so the final complex has a co-ordination number of 4. The complex will also change in charge as the chloride ion is charged. Concentrated hydrochloric acid is used as a source of chloride ions. These reactions are reversible and adding excess water moves the equilibrium back to the hexaqua ion. Cobalt [Co(H2O)6]2+ + 4Cl-(aq) ________ [CoCl4]2-(aq) + 6H2O(l) _______ Copper [Cu(H2O)6]2+ + 4Cl-(aq) ________ [CuCl4]2-(aq) + 6H2O(l) _______ Some more complex reactions Chromium Common Ions: Cr2+ [Ar] , Cr3+[Ar] , Cr6+[Ar] Aqueous potassium chromate is commonly used as an oxidizing agent in organic chemistry. The dichromate ion is reduced to chromium(III) Cr2O72-(aq) + 14H+(aq) +6e- 2Cr3+(aq) + 7H2O(l) ________ _______ The chromate(VI)-dichromate(VI) equilibrium Changing between the orange dichromate(VI) ion, Cr2O72- and the yellow chromate(VI) ion, CrO42- is easy: If you add dilute sulphuric acid to the yellow solution it turns orange. If you add sodium hydroxide solution to the orange solution it turns yellow. The equilibrium reaction at the heart of the interconversion is: . 21 A2 CHEMISTRY NOTES – Transition Metals Reduction of Dichromate ions Aqueous potassium chromate is reduced with ____ and _____________ acid first to the __ oxidation state. +6 to +3 Cr2O72-(aq) + 14H+(aq) +6e- 2Cr3+(aq) + 7H2O(l) Zn (s) Zn2+ + 2e 3Zn (s) + Cr2O72-(aq) + 14H+(aq) 2Cr3+(aq) + 7H2O(l) + 3Zn2+ BUT 2Cr3+(aq) is [Cr(H2O)6]3+ and this undergoes ligand exchange with Cl- ions [Cr(H2O)6]3+(aq) + 2Cl- (aq) [Cr Cl2 (H2O)4]+1(aq) +3 to +2 [Cr Cl2 (H2O)4]+1(aq) ________ [Cr(H2O)6]2+(aq) ______ But if any air is present +2 to +3 [Cr(H2O)6]2+(aq) _____ [Cr Cl2 (H2O)4]+1(aq) _______ Oxidation in alkaline solution +3 to +6 There are 3 stages in oxidizing the Cr(III) to a yellow solution of CrO42-with H2O2. A _____ precipitate forms in the first stage [Cr(H2O)6]3+(aq) + 3OH-(aq) Cr(OH)3(s) + 3H2O(l) The ______ precipitate re-dissolves in excess hydroxide forming a dark green solution Cr(OH)3(aq) + 3OH-(aq) Cr(OH)6(aq) + 3H2O(l) H2O2 is added to the green solution and the mixture is warmed giving the ________ solution. Cr(OH)6(aq) + 3H2O2 (aq) CrO42-(aq) + 8H2O(l) + 2OH- 22 + 2H2O(l) A2 CHEMISTRY NOTES – Transition Metals Cobalt Oxidation in alkaline solution +2 to +3 A blue-green precipitate forms when hydroxide is added to cobalt(II) solution [Co(H2O)6]3+(aq) + 2OH-(aq) [Cr(H2O)4(OH)2](s) + 2H2O(l) Hydrogen peroxide is added to the blue-green precipitate and the mixture is warmed forming a dark brown precipitate. 2[Co(H2O)4(OH)2](s) + H2O2 (aq) 2[Co(H2O)3(OH)3](s) + 2H2O(l) Oxidation by air with ammonia +2 to +3 There are 3 stages in oxidizing the pink Co(II) solution to a dark brown mixture containing hexaamiminebobalt(III). A blue-green hydroxide precipitate forms in the first stage [Cr(H2O)6]3+(aq) + 2NH3(aq) [Co(H2O)4(OH)2](s) + 2NH4+(l) The blue-green precipitate re-dissolves in excess ammonia forming a straw coloured solution of hexamminecobalt(II) solution [Co(H2O)4(OH)2](s) + 6NH3(aq) [Co(NH3)6]2+(aq) + 4H2O(l) +2OH-(aq) The air oxidizes the straw coloured hexamminecobalt(II) solution to yellow hexamminecobalt(III) – the solution actually appears to be dark brown due to the presence of other ions [Co(NH3)6]2+(aq) [Co(NH3)6]3+(aq) + e- 23